The speaker is thinking about someone who is absent, and says the sharpest moment of missing them will come at a particular time of day: dusk by a marsh. The poem never tells us who the person is or why they are gone. All we get is the feeling, and the place where it hits hardest.
The scene is built from small, quiet details. Mist rises and hangs on the reeds, the evening grows cold, and Mallards call out across the water. Each image adds to a sense of emptiness and slow fading light. The cold seems to be physical but also emotional, a chill that comes with being alone. The ducks' cries carry across open space, which suggests distance, perhaps like the gap between the speaker and the person they miss.
The mood is wistful and still. The poem is short and stops without resolution, so the longing seems to linger in the air much like the mist.